420 STELLA B. VINCENT 



With this apparatus the size, form, and relative position of the 

 stimulating object as well as its brightness can be independently 

 varied and controlled. The first experiments were concerned 

 with size — the discrimination of circles of unequal size. The 

 stimuli were at first complex, differing in form, size and bright- 

 ness. Little by little they were changed until the only distinc- 

 tion was that of size. Under these circumstances the birds 

 learned to discriminate between circles whose diameter differed 

 from the standard six centimeters by one-fourth to one-sixth. 

 The control tests left no doubt in the minds of the experimenter 

 that this discrimination was made entirely on the basis of size. 

 He also succeeded in teaching one bird to distinguish between a 

 circle and a triangle of equal area. The bird was unable, how- 

 ever, to make this discrimination when the relative oositions of 

 the forms were changed and he thinks the power depended upon 

 the unequal stimulation of different parts of the retina. 



Hunter (28) in discussing this bit of experimentation, together 

 with some other work, emphasizes the necessity of considering 

 the background in all such investigations. The position is taken 

 that the vision of animals is probably what may be called a 

 pattern vision, that animals see a form within an outlined visual 

 field which must also be varied for perfect control. Real form 

 discrimination he thinks is a more abstract and later mode of 

 perception. 



The dual visual theory assumes that the cones of the retina 

 function for daylight and the rods for twilight vision and that 

 only the cones have the power of adaptation. A study of the 

 eyes of nocturnal animals would seem valuable here. Such 

 studies have been made before. Hess investigated the pupil- 

 motor valence of the dark adapted eye, but did not make any 

 comparisons under conditions of bright adaptation. Katz and 

 Revesz (30) do this in a study of the eyes of owls. They used 

 the light from a Nernst lamp filtered through colored gelatine 

 plates. There was a variable opening for the light which could 

 be easily controlled. Owls which had been kept long in the dark 

 were tested with a very weak light from a small opening. The 

 strength of the pupil-motor reaction was as follows: YG, G, 

 GB&B, R&O, BR. For the two investigators the order was: 

 YG, GB, B, O, R, BR. The human eye makes a difference 

 between GB and B and between O and R which the animals 



